U.S. patent application number 10/369358 was filed with the patent office on 2004-05-13 for pulsed fiber laser cutting system for medical implants.
Invention is credited to Fox, Jason, Jones, Stephen, Kleine, Klaus, Ku, Yu-Chun, Whitney, Brad.
Application Number | 20040089643 10/369358 |
Document ID | / |
Family ID | 32907654 |
Filed Date | 2004-05-13 |
United States Patent
Application |
20040089643 |
Kind Code |
A1 |
Jones, Stephen ; et
al. |
May 13, 2004 |
Pulsed fiber laser cutting system for medical implants
Abstract
An improved expandable stent for implantation in a body lumen,
such as an artery, and an improved method for making it from a
single length of tubing. The stent consists of a plurality of
radially expandable cut cylindrical elements generally aligned on a
common axis and interconnected by one or more interconnective
elements, the elements having a rectangular cross-section from
cut-to-cut. The individual radially expandable cylindrical elements
are disposed in an undulating pattern. The stent is manufactured by
direct laser cutting from a single metal tube using a finely
focused laser beam originating from a diode pumped fiber laser with
an external pulse generator and passing through a coaxial gas jet
structure to impinge on the working surface of the tube as the
linear and rotary velocity of the tube is precisely controlled. To
optimize the cut, the laser parameters may be adjusted and/or the
laser pulse may be shaped.
Inventors: |
Jones, Stephen; (Saugus,
CA) ; Kleine, Klaus; (Los Gatos, CA) ;
Whitney, Brad; (Oakland, CA) ; Ku, Yu-Chun;
(Mountain View, CA) ; Fox, Jason; (Sunnyvale,
CA) |
Correspondence
Address: |
FULWIDER PATTON LEE & UTECHT, LLP
HOWARD HUGHES CENTER
6060 CENTER DRIVE
TENTH FLOOR
LOS ANGELES
CA
90045
US
|
Family ID: |
32907654 |
Appl. No.: |
10/369358 |
Filed: |
February 18, 2003 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10369358 |
Feb 18, 2003 |
|
|
|
09882590 |
Jun 14, 2001 |
|
|
|
6521865 |
|
|
|
|
Current U.S.
Class: |
219/121.72 |
Current CPC
Class: |
B23K 26/123 20130101;
B23K 26/146 20151001; B23K 26/142 20151001; B23K 26/38 20130101;
B23K 2101/06 20180801; A61F 2/91 20130101; B23K 26/14 20130101;
B23K 26/1436 20151001; B23K 26/16 20130101; B23K 26/1438 20151001;
B23K 26/08 20130101; B23K 26/0622 20151001; B23K 2103/50 20180801;
B23K 26/18 20130101; B23K 2103/05 20180801; B23K 2103/42 20180801;
B23K 26/1476 20130101; B23K 26/06 20130101; B23K 26/064
20151001 |
Class at
Publication: |
219/121.72 |
International
Class: |
B23K 026/38 |
Claims
What is claimed:
1. A method of making a stent, comprising: providing a generally
tubular member; providing a fiber laser having, an optical fiber;
and two or more mirrors; pumping said fiber laser using a diode
pump; and cutting a predetermined pattern in the tubular member
using the fiber laser at a cutting speed within a range of about
3.0 to about 5.0 mm/second (about 0.12 to about 0.20
inch/second).
2. The method of claim 1, further comprising gating the pump using
a pulse generator.
3. The method of claim 2, further comprising pulsing the pulse
generator at a frequency in the range of about 1250 to about 1750
Hertz.
4. The method of claim 2, wherein the pulse generator provides
laser pulses having pulse lengths between about 0.05 and about 0.10
milliseconds.
5. The method of claim 1, wherein: providing a fiber laser further
comprises providing a fiber laser incorporating a coaxial gas jet;
and cutting a predetermined pattern in the tubular member further
comprises releasing a compressed gas from the gas jet at a pressure
within a range between about 138 kPa to over about 345 kPa (about
20 psi to over about 50 psi).
6. The method of claim 1, further comprising pumping a liquid
through the tubular member.
7. A method of making a stent, comprising: providing a generally
tubular member; providing a fiber laser having, an optical fiber;
and two or more mirrors; pumping said fiber laser using a diode
pump; cutting a predetermined pattern in the tubular member using
the fiber laser; and shaping the laser during a laser pulse.
8. The method of claim 7, further comprising gating the pump using
a pulse generator.
9. The method of claim 8, wherein the pulse generator provides
laser pulses having pulse lengths between about 0.02 and about 0.20
milliseconds.
10. The method of claim 9, wherein shaping the pulse includes
modifying the pulse in increments as small as about 0.005
milliseconds.
11. The method of claim 7, further comprising pumping a liquid
through the tubular member.
Description
[0001] This application is a continuation-in-part of co-pending
U.S. application Ser. No. 09/882,590, filed Jun. 14, 2001, for a
"Pulsed Fiber Laser Cutting System for Medical Implants," the
entire contents of which are hereby incorporated by reference.
BACKGROUND OF THE INVENTION
[0002] This invention relates generally to improvements in the
manufacture of expandable metal stents and, more particularly, to
new and improved methods and apparatus for direct laser cutting of
metal stents and providing stents of enhanced structural
quality.
[0003] Stents are expandable endoprosthesis devices which are
adapted to be implanted into a patient's body lumen, such as a
blood vessel, to maintain the patency of the vessel. These devices
are typically used in the treatment of atherosclerotic stenosis in
blood vessels and the like.
[0004] In the medical arts, stents are generally tubular-shaped
devices which function to hold open a segment of a blood vessel or
other anatomical lumen. They are particularly suitable for use to
support and hold back a dissected arterial lining which can occlude
the fluid passageway.
[0005] Various means have been provided to deliver and implant
stents. One method frequently described for delivering a stent to a
desired intraluminal location includes mounting the expandable
stent on an expandable member, such as a balloon; provided on the
distal end of an intravascular catheter, advancing the catheter to
the desired location within the patient's body lumen, inflating the
balloon on the catheter to expand the stent into a permanent
expanded condition and then deflating the balloon and removing the
catheter.
[0006] One example of a particularly useful expandable stent is a
stent which is relatively flexible along its longitudinal axis to
facilitate delivery through tortuous body lumens, but which is
stiff and stable enough radially in an expanded condition to
maintain the patency of a body lumen such as an artery when
implanted within the lumen. Such a desirable stent typically
includes a plurality of radially expandable cylindrical elements
which are relatively independent in their ability to expand and to
flex relative to one another. The individual radially expandable
cylindrical elements of the stent are precisely dimensioned so as
to be longitudinally shorter than their own diameters.
Interconnecting elements or struts extending between adjacent
cylindrical elements provide increased stability and are positioned
to prevent warping of the stent when it is expanded. The resulting
stent structure is a series of radially expandable cylindrical
elements which are spaced longitudinally close enough so that small
dissections in the wall of a body lumen may be pressed back into
position against the luminal wall, but not so close as to
compromise the longitudinal flexibility of the stent. The
individual cylindrical elements may rotate slightly relative to
adjacent cylindrical elements without significant deformation,
cumulatively giving a stent which is flexible along its length and
about its longitudinal axis, but is still very stiff in the radial
direction in order to resist collapse.
[0007] The prior art stents generally have a precisely laid out
circumferential undulating pattern, e.g. serpentine. The transverse
cross-section of the undulating component of the cylindrical
element is relatively small and preferably has an aspect ratio of
about two to one to about one-half-to-one. A one-to-one aspect
ratio also has been found particularly suitable. The open
reticulated structure of the stent allows for the perfusion of
blood over a large portion of the arterial wall which can improve
the healing and repair of a damaged arterial lining.
[0008] The radial expansion of the expandable cylinder deforms the
undulating pattern similar to changes in a waveform which result
from decreasing the waveform's amplitude and the frequency. In the
case of a balloon-expandable stent, such as one made from stainless
steel, the cylindrical structures of the stent are plastically
deformed when expanded so that the stent will remain in the
expanded condition and, therefore, they must be sufficiently rigid
when expanded to prevent their collapse in use. During expansion of
the stent, portions of the undulating pattern may tip outwardly
resulting in projecting members on the outer surface of the
expanded stent. These projecting members tip radially outwardly
from the outer surface of the stent and embed in the vessel wall
and help secure the expanded stent so that it does not move once it
is implanted.
[0009] The elements or struts which interconnect adjacent
cylindrical elements should have a precisely defined transverse
cross-section similar to the transverse dimensions of the
undulating components of the expandable cylindrical elements. The
interconnecting elements may be formed as a unitary structure with
the expandable cylindrical elements from the same intermediate
product, such as a tubular element, or they may be formed
independently and connected by suitable means, such as by welding
or by mechanically securing the ends of the interconnecting
elements to the ends of the expandable cylindrical elements.
Preferably, all of the interconnecting elements of a stent are
joined at either the peaks or the valleys of the undulating
structure of the cylindrical elements which form the stent. In this
manner, there is minimal or no shortening of the stent upon
expansion.
[0010] The number and location of elements interconnecting adjacent
cylindrical elements can be varied in order to develop the desired
longitudinal flexibility in the stent structure both in the
unexpanded, as well as the expanded condition. These properties are
important to minimize alteration of the natural physiology of the
body lumen into which the stent is implanted and to maintain the
compliance of the body lumen which is internally supported by the
stent. Generally, the greater the longitudinal flexibility of the
stent, the easier and the more safely it can be delivered to the
implantation site.
[0011] It will be apparent from the foregoing that conventional
stents are very high precision, relatively fragile devices and,
ideally, the most desirable metal stents incorporate a fine
precision structure cut from a very small diameter, thin-walled
cylindrical tube. In this regard, it is extremely important to make
precisely dimensioned, smooth, narrow cuts in the stainless tubes
in extremely fine geometries without damaging the narrow struts
that make up the stent structure. While the various laser cutting
processes and chemical etching, heretofore utilized by the prior
art to form such expandable metal stents, have been adequate,
improvements have been sought to provide stents of enhanced
structural quality in terms of resolution, reliability and
yield.
[0012] Accordingly, those concerned with the development,
manufacture and use of metal stents have long recognized the need
for improved manufacturing processes for such stents. The present
invention fulfills these needs.
SUMMARY OF THE INVENTION
[0013] Briefly, and in general terms, the present invention
provides a new and improved method and apparatus for direct laser
cutting of metal stents enabling greater precision, reliability,
structural integrity and overall quality, while minimizing burrs,
slag or other imperfections which might otherwise hamper stent
integrity and performance.
[0014] The present invention provides an improved system for
producing metal stents with a fine precision structure cut from a
small diameter, thin-walled, cylindrical tube. The tubes are
typically made of stainless steel and are fixtured under a laser
and positioned utilizing CNC (computer numerical control) to
generate a very intricate and precise pattern. Due to the thin-wall
and the small geometry of the stent pattern, it is necessary to
have very precise control of the laser, its power level, and the
precise positioning of the laser cutting path.
[0015] In one embodiment of the invention, in order to minimize the
heat input, which prevents thermal distortion, uncontrolled burnout
of the metal, and metallurgical damage due to excessive heat, a
diode pumped fiber laser is utilized. Further, an external pulse
generator is employed so that laser pulses having pulse lengths
between 0.02 and 0.50 milliseconds are achieved at a frequency
range of 100 to 3000 Hz. With these parameters, it is possible to
make smooth, narrow cuts in the stainless steel tubes in very fine
geometries without damaging the narrow struts that make up the
stent structure.
[0016] In addition to the laser and the precision CNC positioning
equipment, a coaxial gas jet is also utilized to provide for
additional heat reduction in the workpiece by introducing a gas
stream that surrounds the focused laser beam and is, directed along
the beam axis. The coaxial gas jet nozzle is centered around the
focused beam with approximately 0.25 mm (0.010 inch) between the
tip of the nozzle and the tubing. The jet may be pressurized at
over 345 kPa (50 psi). In one embodiment of the invention, the jet
is pressurized with oxygen at 138 kPa (20 psi) and is directed at
the tube with the focused laser beam exiting the tip of the nozzle.
The oxygen reacts with the metal to assist in the cutting process
very similar to oxyacetylene cutting. The focused laser beam acts
as an ignition source and controls the reaction of the oxygen with
the metal. In this manner, it is possible to cut the material with
a very fine kerf with precision. In order to prevent burning by the
beam and/or molten slag on the far wall of the tube inside
diameter, a stainless steel mandrel is placed inside the tube and
is allowed to roll on the bottom of the tube as the pattern is cut.
This acts as a beam/debris block protecting the far wall inside
diameter.
[0017] Additional heat reduction is provided by a liquid
introduction system which pumps liquid, such as water, through the
tube while it is being laser cut. As the liquid flows through the
tube, it removes heat and slag caused by the laser cutting
process.
[0018] In one embodiment of the invention, the operational
parameters of the diode-pumped fiber laser may be adjusted to yield
optimal cutting results, characterized by low surface roughness at
the edges and a minimal heat-affected zone. For example, the pulse
frequency may be adjusted to within a range of about 1250 to about
1750 Hz and the cutting speed may be adjusted to within a range of
about 3.0 to about 5.0 mm/second (about 0.12 to about 0.20
inches/second) to attain low surface roughness. Also, a laser pulse
length within a range of about 0.05 to about 0.10 milliseconds may
be used to minimize the heat-affected zone.
[0019] In another embodiment of the invention, the diode-pumped
fiber laser may include an arbitrary function generator to control
the pump diode, thereby enabling the diode-pumped fiber laser to
perform pulse shaping. In one embodiment, the pulse is shaped
between a range of about 0.02 to about 0.20 milliseconds. The
arrangement of the pump diode permits shaping of the laser pulse
with high resolution, such as increments of about 0.005
milliseconds. The ability to shape the laser pulse permits
materials to be cut with tailored laser pulses to provide optimal
cutting results.
[0020] The cutting process which utilizes oxygen with the finely
focused diode pumped fiber laser beam results in a very narrow kerf
(approximately 0.013 mm or 0.0005 inch) with the molten slag
resolidifying along the cut. This traps the cutout scrap of the
pattern and requires further processing. In order to remove the
slag debris from the cut and allow the scrap to be removed from the
remaining stent pattern, it is desirable to soak the cut tube in a
solution of HCl for a selected time and temperature. Before it is
soaked, the tube is placed in a bath of alcohol/water solution and
ultrasonically cleaned for approximately one minute to remove the
loose debris left from the cutting operation. After soaking, the
tube is then ultrasonically cleaned in the heated HCl for a period
of time dependent upon the wall thickness. To prevent cracking or
breaking of the struts attached to the material left at the two
ends of the stent pattern due to harmonic oscillations induced by
the ultrasonic cleaner, a mandrel is placed down the center of the
tube during the cleaning/scrap removal process. Upon completion of
this process, the stent structures are rinsed in water and are then
ready for electropolishing.
[0021] Hence, the new and improved method and apparatus for direct
laser cutting of metal stents, in accordance with the present
invention, makes accurate, reliable, high resolution, expandable
stents with patterns having smooth, narrow cuts and very fine
geometries.
[0022] The above and other objects and advantages of this invention
will be apparent from the following more detailed description when
taken in conjunction with the accompanying drawings of exemplary
embodiments.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is an elevational view, partially in section, of a
stent embodying features of the invention which is mounted on a
delivery catheter and disposed within an artery.
[0024] FIG. 2 is an elevational view, partially in section, similar
to that shown in FIG. 1, wherein the stent is expanded within an
artery.
[0025] FIG. 3 is an elevational view, partially in section, showing
the expanded stent within the artery after withdrawal of the
delivery catheter.
[0026] FIG. 4 is a perspective view of a stent embodiment in an
unexpanded state, with one end of the stent being shown in an
exploded view to illustrate the details thereof.
[0027] FIG. 5 is a plan view of a flattened section of a stent of
the invention which illustrates the undulating pattern of the stent
as shown in FIG. 4.
[0028] FIG. 5a is a sectional view taken along the line 5a-5a in
FIG. 5.
[0029] FIG. 6 is a schematic representation of equipment for
selectively cutting the tubing in the manufacture of stents, in
accordance with the present invention.
[0030] FIG. 7 is an elevational view of a system for cutting an
appropriate pattern by laser in a metal tube to form a stent, in
accordance with the present invention.
[0031] FIG. 8 is a plan view of the laser head and optical delivery
subsystem for the laser cutting system shown in FIG. 7.
[0032] FIG. 8a is a partial enlarged view depicting the parallel
mirrors.
[0033] FIG. 9 is an elevational view of a coaxial gas jet, rotary
collet, tube support and beam blocking apparatus for use in the
laser cutting system shown in FIG. 7.
[0034] FIG. 10 is a sectional view taken along the line 10-10 in
FIG. 9.
[0035] FIG. 11 is an elevational view of a coaxial gas jet, rotary
collet, tube support, beam blocking apparatus and liquid
introduction system.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] Referring now to the drawings, and particularly FIG. 1,
there is shown a stent 10 that is mounted onto a delivery catheter
11. The stent is a high precision patterned tubular device. The
stent typically comprises a plurality of radially expanded
cylindrical elements 12 disposed generally coaxially and
interconnected by elements 13 disposed between adjacent cylindrical
elements. The delivery catheter has an expandable portion or
balloon 14 for expanding of the stent within an artery 15.
[0037] The typical delivery catheter 11 onto which the stent 10 is
mounted is essentially the same as a conventional balloon
dilatation catheter for angioplasty procedures. The balloon 14 may
be formed of suitable materials such as polyethylene, polyethylene
terephthalate, polyvinyl chloride, nylon and ionomers such as
Surlyn.RTM. manufactured by the Polymer Products Division of the Du
Pont Company. Other polymers may also be used. In order for the
stent to remain in place on the balloon during delivery to the site
of the damage within the artery 15, the stent is compressed onto
the balloon. A retractable protective delivery sheath 20 may be
provided to further ensure that the stent stays in place on the
expandable portion of the delivery catheter and prevent abrasion of
the body lumen by the open surface of the stent during delivery to
the desired arterial location. Other means for securing the stent
onto the balloon may also be used, such as providing collars or
ridges on the ends of the working portion, i.e., the cylindrical
portion, of the balloon.
[0038] The delivery of the stent 10 is accomplished in the
following manner. The stent is first mounted onto the inflatable
balloon 14 on the distal extremity of the delivery catheter 11. The
balloon is slightly inflated to secure the stent onto the exterior
of the balloon. The catheter-stent assembly is introduced within
the patient's vasculature in a conventional Seldinger technique
through a guiding catheter (not shown). A guide wire 18 is disposed
across the target arterial section and then the catheter/stent
assembly is advanced over the guide wire within the artery 15 until
the stent is positioned in the target area. The balloon of the
catheter is expanded, expanding the stent against the artery, which
is illustrated in FIG. 2. While not shown in the drawing, the
artery is preferably expanded slightly by the expansion of the
stent to seat or otherwise fix the stent to prevent movement. In
some circumstances during the treatment of stenotic portions of an
artery, the artery may have to be expanded considerably in order to
facilitate passage of blood or other fluid therethrough.
[0039] The stent 10 serves to hold open the artery 15 after the
catheter 11 is withdrawn, as illustrated by FIG. 3. Due to the
formation of the stent from an elongated tubular member, the
undulating component of the cylindrical elements of the stent is
relatively flat in transverse cross-section, so that when the stent
is expanded, the cylindrical elements are pressed into the wall of
the artery and, as a result, do not interfere with the blood flow
through the artery. The cylindrical elements 12 of the stent, which
are pressed into the wall of the artery, will eventually be covered
with endothelial cell growth which further minimizes blood flow
interference. The undulating portion of the cylindrical elements
provides good tacking characteristics to prevent stent movement
within the artery. Furthermore, the closely spaced cylindrical
elements at regular intervals provide uniform support for the wall
of the artery and, consequently, are well adapted to tack up and
hold in place small flaps or dissections in the wall of the
artery.
[0040] FIG. 4 is an enlarged perspective view of the stent 10 shown
in FIG. 1 with one end of the stent shown in an exploded view to
illustrate in greater detail the placement of interconnecting
elements 13 between adjacent radially expandable cylindrical
elements 12. Each pair of interconnecting elements on one side of a
cylindrical element are preferably placed to allow maximum
flexibility for a stent. In the embodiment shown in FIG. 4, the
stent has three interconnecting elements between adjacent radially
expandable cylindrical elements that are 120.degree. apart. Each
pair of interconnecting elements on one side of a cylindrical
element are offset radially 60.degree. from the pair on the other
side of the cylindrical element. The alternation of the
interconnecting elements results in a stent which is longitudinally
flexible in essentially all directions. The primary flexibility of
this stent design derives from the cylindrical elements, while the
interconnecting element actually reduces the overall stent
flexibility. Various configurations for the placement of
interconnecting elements are possible. However, as previously
mentioned, all of the interconnecting elements of an individual
stent should be secured to either the peaks or valleys of the
undulating structural elements in order to prevent shortening of
the stent during the expansion thereof.
[0041] The number of undulations may also be varied to accommodate
placement of interconnecting elements 13, e.g., at the peaks of the
undulations or along the sides of the undulations as shown in FIG.
5.
[0042] As best observed in FIGS. 4 and 5, the cylindrical elements
12 are in the form of a serpentine pattern 30. As previously
mentioned, each cylindrical element is connected by interconnecting
elements 13. The serpentine pattern is made up of a plurality of
U-shaped members 31, W-shaped members 32, and Y-shaped members 33,
each having a different radius so that expansion forces are more
evenly distributed over the various members.
[0043] The aforedescribed illustrative stent 10 and similar stent
structures can be made in many ways. However, the preferred method
of making the stent is to cut a thin-walled tubular member 16, such
as stainless steel tubing, to remove portions of the tubing in the
desired pattern for the stent, leaving relatively untouched the
portions of the metallic tubing which are to form the stent.
[0044] The tubing 16 may be made of a suitable biocompatible
material such as stainless steel. For example, the stainless steel
tubing may be Alloy type 316L SS, Special Chemistry per ASTM
F138-92 or ASTM F139-92 grade 2. Special Chemistry of type 316L per
ASTM F138-92 or ASTM F139-92 Stainless Steel for surgical implants
in weight percent is as follows:
1 Carbon (C) 0.03% max. Manganese (Mn) 2.00% max. Phosphorous (P)
0.025% max. Sulphur (5) 0.010% max. Silicon (Si) 0.75% max.
Chromium (Cr) 17.00-19.00% Nickel (Ni) 13.00-15.50% Molybdenum (Mo)
2.00-3.00% Nitrogen (N) 0.10% max. Copper (Cu) 0.50% max. Iron (Fe)
Balance
[0045] The present invention pulsed fiber laser cutting system can
be used to cut any stent pattern and virtually any stent material.
The invention is not limited to cutting tubular members made from
stainless steel. For example, tubular members being formed from any
number of metals are possible, including cobalt-chromium, titanium,
nickel-titanium, tantalum, gold, platinum,
nickel-titanium-platinum, and other similar metal alloys.
[0046] The stent diameter is very small, so the tubing from which
it is made must necessarily also have a small diameter. For
coronary applications, typically, the stent has an outer diameter
on the order of about 1.5 mm (0.06 inch) in the unexpanded
condition, equivalent to the tubing from which the stent is made,
and can be expanded to an outer diameter of 2.5 mm (0.100 inch) or
more. The wall thickness of the tubing is about 0.08 mm (0.003
inch).
[0047] In accordance with the present invention, it is preferred to
cut the tubing 16 in the desired pattern by means of a
machine-controlled laser as illustrated schematically in FIG. 6. A
machine-controlled laser cutting system is generally depicted as
disclosed in U.S. Pat. No. 5,780,807 to Richard J. Saunders and is
incorporated herein by reference. The tubing 21 is placed in a
rotatable collet fixture 22 of a machine-controlled apparatus 23
for positioning the tubing relative to the laser 24. According to
machine-encoded instructions, the tubing is rotated and moved
longitudinally relative to the laser, which is also
machine-controlled. The laser selectively removes the material from
the tubing by ablation and a pattern is cut into the tube. The tube
is therefore cut into the discrete pattern of the finished
stent.
[0048] The process of cutting a pattern for the stent into the
tubing 16 is automated except for loading and unloading the length
of tubing. Referring again to FIG. 6, the cutting may be done, for
example, using a CNC-opposing collet fixture 22 for axial rotation
of the length of tubing, in conjunction with a CNC X/Y table 25 for
movement of the length of tubing axially relative to the
machine-controlled laser, as described. The program for control of
the apparatus is dependent on the particular configuration used and
the pattern to be ablated in the tubing.
[0049] Referring now to FIGS. 7-10, there is shown a process and
apparatus, in accordance with the present invention, for producing
metal stents with a fine precision structure cut from a small
diameter thin-walled cylindrical tube 16. Cutting a fine structure
(0.889 mm web width (0.0035 inch)) requires precise laser focusing
and minimal heat input. In order to satisfy these requirements, an
improved laser technology has been adapted to this micro-machining
application according to the present invention.
[0050] The diode pumped fiber laser 40, as illustrated in FIG. 7,
is comprised of an optical fiber 42 and a diode pump 43 integrally
mounted coaxial to the optical fiber. In one embodiment, as shown
in FIG. 8a, two mirrors 46, 47 are mounted within the optical fiber
such that the mirrors are parallel to one another and normal to the
central axis of the optical fiber. The two mirrors are spaced apart
by a fixed distance creating an area within the optical fiber
between the mirrors called the active region. This type of fiber
laser is typically available from SDL and is rated at 23 watts.
[0051] A pulse generator 44 is mounted external to the diode pumped
fiber laser 40 in the area of the horizontal mounting surface 45.
The pulse generator provides restricted and more precise control of
the laser's output by gating the diode pump 43. By employing the
external pulse generator, laser pulses having pulse lengths between
about 0.02 and 0.50 milliseconds are achieved at a frequency range
of about 100 to 3000 Hz. The pulse generator is a conventional
model obtainable from any number of manufacturers and operates on
standard 110 volt AC.
[0052] The diode pumped fiber laser 40 operates with low-frequency,
pulsed wavelengths in order to minimize the heat input into the
stent structure, which prevents thermal distortion, uncontrolled
burn out of the metal, and metallurgical damage due to excessive
heat, and thereby produce a smooth, debris-free cut. In use, the
diode pump 43 generates light energy at the proximal end of the
optical fiber. Initially, the light energy is pulsed by the
external pulse generator 44. Next, the pulsed light energy
transmissions pass along the optical fiber 42 and through the first
mirror. Between the first and second mirror the light is resonated
in the fiber laser's active region. Then, the light passes through
the second mirror and continues along the length of the optical
fiber. Finally, the light exits the distal end of the fiber and
ultimately impinges upon the workpiece.
[0053] It will be appreciated by those skilled in the art that, in
use, the diode pumped fiber laser 40 of the present invention is
low in maintenance because it does not require a flash lamp or
realignment as with conventional laser cutting systems. The fiber
laser system is also more efficient and maintenance-free due to
being air cooled, as opposed to water cooled, and operating on
standard 110 volts AC power. Further, the fiber laser system may
occupy as little as one-third the space occupied by conventional
laser systems, thereby allowing for optimization of the square
footage of manufacturing facilities.
[0054] The diode pumped fiber laser 40 incorporates a coaxial gas
jet 50 and nozzle 51 that helps to remove debris from the kerf and
cools the region where the beam interacts with the material as the
beam cuts and vaporizes the metal. The coaxial gas jet nozzle (0.46
mm diameter (0.018 inch)) is centered around the focused beam with
approximately 2.54 mm (0.010 inch) between the tip of the nozzle
and the tubing 16. In many cases, the gas utilized in the jets may
be reactive or non-reactive (inert). In the case of reactive gas,
oxygen or compressed air may be used. In one embodiment of the
invention, the jet may be pressurized with the gas at over 345 kPa
(50 psi).
[0055] In one embodiment, the jet is pressurized with oxygen at 138
kPa (20 psi) and is directed at the tube 16 with the focused laser
beam 55 exiting the tip 53 of the nozzle (0.457 mm diameter (0.018
inch)). The oxygen reacts with the metal to assist in the cutting
process very similar to oxyacetylene cutting. The focused laser
beam acts as an ignition source and controls the reaction of the
oxygen with the metal. In this manner, it is possible to cut the
material with a very fine kerf with precision.
[0056] In other embodiments of the present invention, compressed
air may be used in the gas jet 50 since it offers more control of
the material removed and reduces the thermal effects of the
material itself. Inert gas such as argon, helium, or nitrogen can
be used to eliminate any oxidation of the cut material. The result
is a cut edge with no oxidation, but there is usually a tail of
molten material that collects along the exit side of the gas jet
that must be mechanically or chemically removed after the cutting
operation.
[0057] In either case, it is also necessary to block the laser beam
55 as it cuts through the top surface of the tube and prevent the
beam, along with the molten metal and debris from the cut, from
impinging on the inside opposite surface of the tube 16. To this
end, a stainless steel mandrel 56 (approx. 0.864 mm diameter (0.034
inch)) is placed inside the tube and is allowed to roll on the
bottom of the tube 16 as the pattern is cut. This acts as a
beam/debris block protecting the far wall inner diameter.
[0058] Hence, the diode pumped fiber laser system 40 of the present
invention enables the machining of narrow kerf widths while
minimizing the heat input into the material. Thus, it is possible
to make smooth, narrow cuts in the tube 16 in very fine geometries
without damaging the narrow struts that make up the stent
structure.
[0059] Cutting a fine structure also requires the ability to
manipulate the tube with precision. The positioning of the tubular
structure requires the use of precision CNC equipment such as that
manufactured and sold by Anorad Corporation. In addition, a unique
rotary mechanism has been provided that allows the computer program
to be written as if the pattern were being cut from a flat sheet.
This allows both circular and linear interpolation to be utilized
in programming. Since the finished structure of the stent is very
small, a precision drive mechanism is required that supports and
drives both ends of the tubular structure as it is cut. Since both
ends are driven, they must be aligned and precisely synchronized,
otherwise the stent structure would twist and distort as it is
being cut. The processing speed, or cutting speed, of the laser as
it is cutting the tube may be altered depending on the material of
the tube and the finish which is sought for the completed stent.
The processing speed may also be adjusted to minimize the
heat-affected zone. In one embodiment, the processing speed of the
laser as it is cutting the tube may be about 3.0 to about 5.0
mm/sec (about 0.12 to about 0.20 inches/sec), however, faster or
slower speeds may also be implemented as desired.
[0060] In one embodiment of the invention, the operational
parameters of the diode-pumped fiber laser may be adjusted to yield
optimal cutting results, characterized by low surface roughness at
the edges and a minimal heat-affected zone. The parameters which
may be adjusted to attain the desirable results include pulse
frequency, pulse length, peak pulse power, average power, type of
assist gas, pressure of the assist gas, and processing speed. For
example, to attain low surface roughness when cutting stainless
steel, one embodiment of the invention includes adjusting the peak
pulse power to a low level, adjusting the pulse frequency to within
a range of about 1250 to about 1750 Hz and adjusting the cutting
speed to within a range of about 3.0 to about 5.0 mm/second (about
0.12 to about 0.20 inches/second). The cutting speed is dependent
on the frequency setting. For example, a slower cutting speed is
selected for a lower frequency and a faster cutting speed is
selected for a higher frequency. To minimize the heat-affected zone
while cutting stainless steel, the gas pressure may be greater than
345 kPa (50 psi) and a laser pulse length within a range of about
0.05 to about 0.10 milliseconds may be used. The lower limit of
about 0.05 milliseconds the pulse length is used to facilitate a
low surface roughness and to keep the peak pulse power low.
Further, the average power may be adjusted to the minimum level
necessary to attain a desirable cut. Minimizing the average power
facilitates minimized heat input into the work piece and attainment
of low peak pulse power.
[0061] The diode-pumped fiber laser may include an arbitrary
function generator to control the pump diode which enables the
diode-pumped fiber laser to perform pulse shaping, or modification
during a pulse. In one embodiment, the pulse is shaped between a
range of about 0.02 to about 0.20 milliseconds. The arrangement of
the pump diode permits shaping of the laser pulse with high
resolution, such as increments of about 0.005 milliseconds. The
ability to shape the laser pulse permits materials to be cut with
tailored laser pulses to provide optimal cutting results. The
tailored pulses may be selected to provide smaller heat-affected
zones and/or improved surface roughness and is particularly useful
for cutting materials which are tough to polish, such as
cobalt-chromium and platinum alloys.
[0062] As discussed above, the cutting process utilizing the diode
pumped fiber laser and gas jet cooling results in a very narrow
kerf (approximately 0.0005 inch (0.013 mm)) with the molten slag
resolidifying along the cut. This traps the cutout scrap of the
pattern requiring further processing. In order to remove the slag
debris from the cut allowing the scrap to be removed from the
remaining stent pattern, it is necessary to soak the cut tube 16 in
a solution of HCl for approximately 8 minutes at a temperature of
approximately 55.degree. C. (131.degree. F.). Before it is soaked,
the tube is placed in a bath of alcohol/water solution and
ultrasonically cleaned for approximately one minute to remove the
loose debris left from the cutting operation. After soaking, the
tube is then ultrasonically cleaned in the heated HCl for 1-4
minutes depending upon the wall thickness. To prevent
cracking/breaking of the struts attached to the material left at
the two ends of the stent pattern due to harmonic oscillations
induced by the ultrasonic cleaner, a mandrel is placed down the
center of the tube during the cleaning/scrap removal process. Upon
completion of this process, the stent structures are rinsed in
water. They are now ready for electropolishing.
[0063] The stents are preferably electrochemically polished in an
acidic aqueous solution such as a solution of ELECTRO-GLO #300,
sold by the ELECTRO-GLO Co., Inc. in Chicago, Ill., which is a
mixture of sulfuric acid, carboxylic acids, phosphates, corrosion
inhibitors, and a biodegradable surface active agent. The bath
temperature is maintained at about 43 to about 57.degree. C. (about
110 to about 135.degree. F.) and the current density is about 0.062
to about 0.232 amps/cm.sup.2 (about 0.4 to about 1.5
amps/in.sup.2). Cathode to anode area should be at least about two
to one. The stents may be further treated if desired, for example
by applying a biocompatible coating. Other solutions and processes
to electrochemically polish laser cut stents are known in the
art.
[0064] Referring to FIG. 11, a liquid introduction system may be
used to facilitate cooling and cleaning of the tube 16 during the
laser cutting procedure. The liquid introduction system pumps a
liquid, such as water, through the tube during the laser cutting
process. As the liquid flows through the tube, it removes heat and
slag caused by the laser cutting process. The liquid may be
introduced into the tube 16 via a hose 60 or other tubular member
coupled to the rotatable collet fixture 22, such that the liquid
enters the tube from the side supported by the collet and flows out
of the opposite end, or by other methods which are well known in
the art. The pressure of the liquid may be within a range of about
6.89 to about 689 kPa (about 1 to about 100 psi). Although water
performs well in removing heat from the tube during the laser
cutting process, other nonflammable liquids may alternatively be
used to remove the heat and slag.
[0065] Direct laser cutting produces edges which are essentially
perpendicular to the axis of the laser cutting beam, in contrast
with chemical etching and the like which produce pattern edges
which are angled. Hence, the laser cutting process of the present
invention essentially provides stent cross-sections, from
cut-to-cut, which are square or rectangular, rather than
trapezoidal; see FIG. 5a. The resulting stent structure provides
superior performance.
[0066] It will be apparent from the foregoing that the present
invention provides a new and improved method and apparatus for
direct laser cutting of metal stents, enabling greater precision,
reliability, structural integrity and overall quality,
substantially without burrs, slag or other imperfections which
might otherwise hamper stent integrity and performance. While the
invention has been illustrated and described herein in terms of its
use relative to an intravascular stent for use in arteries and
veins, it will be apparent to those skilled in the art that the
invention can be used to manufacture stents for other uses, such as
the biliary tract, or to expand prostatic urethras in cases of
prostate hyperplasia, and to manufacture other medical products
requiring precision micro-machining.
[0067] Therefore, while particular forms of the invention have been
illustrated and described, various modifications can be made
without departing from the spirit and scope of the invention.
Accordingly, it is not intended that the invention be limited,
except as by the appended claims.
* * * * *